An LED device for a back light module typically includes a plurality of LED dies and a fluorescent layer covering the LED dies to obtain a surface light source. The LED die usually has a light output angle about 120°, which has an uneven distribution of light field with high light intensity at center thereof and low light intensity at periphery thereof. Therefore, a diffuser plate is always applied to increase the light output angle and distribute the light evenly at center and at periphery thereof. However, the diffuser plate which has a high diffusion capability also has a poor penetration capability. While traveling in the diffuser plate, more light emitted from the LED die will be absorbed. Thus, a luminous efficiency of the LED device will be reduced when the light emitted from the LED die travels in the diffuser plate and is reflected again and again therein and is partially absorbed by the diffuser plate.